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Vlastimil Babka 700d2e9a36 mm, page_alloc: reduce page alloc/free sanity checks 
Historically, we have performed sanity checks on all struct pages being
allocated or freed, making sure they have no unexpected page flags or
certain field values.  This can detect insufficient cleanup and some cases
of use-after-free, although on its own it can't always identify the
culprit.  The result is a warning and the "bad page" being leaked.

The checks do need some cpu cycles, so in 4.7 with commits 479f854a20
("mm, page_alloc: defer debugging checks of pages allocated from the PCP")
and 4db7548ccb ("mm, page_alloc: defer debugging checks of freed pages
until a PCP drain") they were no longer performed in the hot paths when
allocating and freeing from pcplists, but only when pcplists are bypassed,
refilled or drained.  For debugging purposes, with CONFIG_DEBUG_VM enabled
the checks were instead still done in the hot paths and not when refilling
or draining pcplists.

With 4462b32c92 ("mm, page_alloc: more extensive free page checking with
debug_pagealloc"), enabling debug_pagealloc also moved the sanity checks
back to hot pahs.  When both debug_pagealloc and CONFIG_DEBUG_VM are
enabled, the checks are done both in hotpaths and pcplist refill/drain.

Even though the non-debug default today might seem to be a sensible
tradeoff between overhead and ability to detect bad pages, on closer look
it's arguably not.  As most allocations go through the pcplists, catching
any bad pages when refilling or draining pcplists has only a small chance,
insufficient for debugging or serious hardening purposes.  On the other
hand the cost of the checks is concentrated in the already expensive
drain/refill batching operations, and those are done under the often
contended zone lock.  That was recently identified as an issue for page
allocation and the zone lock contention reduced by moving the checks
outside of the locked section with a patch "mm: reduce lock contention of
pcp buffer refill", but the cost of the checks is still visible compared
to their removal [1].  In the pcplist draining path free_pcppages_bulk()
the checks are still done under zone->lock.

Thus, remove the checks from pcplist refill and drain paths completely.
Introduce a static key check_pages_enabled to control checks during page
allocation a freeing (whether pcplist is used or bypassed). The static
key is enabled if either is true:

- kernel is built with CONFIG_DEBUG_VM=y (debugging)
- debug_pagealloc or page poisoning is boot-time enabled (debugging)
- init_on_alloc or init_on_free is boot-time enabled (hardening)

The resulting user visible changes:
- no checks when draining/refilling pcplists - less overhead, with
  likely no practical reduction of ability to catch bad pages
- no checks when bypassing pcplists in default config (no
  debugging/hardening) - less overhead etc. as above
- on typical hardened kernels [2], checks are now performed on each page
  allocation/free (previously only when bypassing/draining/refilling
  pcplists) - the init_on_alloc/init_on_free enabled should be sufficient
  indication for preferring more costly alloc/free operations for
  hardening purposes and we shouldn't need to introduce another toggle
- code (various wrappers) removal and simplification

[1] https://lore.kernel.org/all/68ba44d8-6899-c018-dcb3-36f3a96e6bea@sra.uni-hannover.de/
[2] https://lore.kernel.org/all/63ebc499.a70a0220.9ac51.29ea@mx.google.com/

[akpm@linux-foundation.org: coding-style cleanups]
[akpm@linux-foundation.org: make check_pages_enabled static]
Link: https://lkml.kernel.org/r/20230216095131.17336-1-vbabka@suse.cz
Reported-by: Alexander Halbuer <halbuer@sra.uni-hannover.de>
Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Kees Cook <keescook@chromium.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
2023-03-28 16:20:06 -07:00
arch - Properly clear perf event status tracking in the AMD perf event 2023-03-26 09:13:35 -07:00
block block: remove obsolete config BLOCK_COMPAT 2023-03-16 09:35:44 -06:00
certs Kbuild updates for v6.3 2023-02-26 11:53:25 -08:00
crypto asymmetric_keys: log on fatal failures in PE/pkcs7 2023-03-21 16:23:56 +00:00
Documentation USB/Thunderbolt driver fixes for 6.3-rc4 2023-03-26 10:22:44 -07:00
drivers USB/Thunderbolt driver fixes for 6.3-rc4 2023-03-26 10:22:44 -07:00
fs fsdax: dedupe should compare the min of two iters' length 2023-03-28 15:24:32 -07:00
include - Do the delayed RCU wakeup for kthreads in the proper order so that 2023-03-26 09:06:20 -07:00
init bootconfig: Change message if no bootconfig with CONFIG_BOOT_CONFIG_FORCE=y 2023-03-22 22:21:43 +09:00
io_uring block-6.3-2023-03-24 2023-03-24 14:10:39 -07:00
ipc Merge branch 'work.namespace' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2023-02-24 19:20:07 -08:00
kernel - Fix a corner case where vruntime of a task is not being sanitized 2023-03-26 09:18:30 -07:00
lib lib/Kconfig.debug: correct help info of LOCKDEP_STACK_TRACE_HASH_BITS 2023-03-28 15:24:31 -07:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm mm, page_alloc: reduce page alloc/free sanity checks 2023-03-28 16:20:06 -07:00
net nfsd-6.3 fixes: 2023-03-25 13:32:43 -07:00
rust Rust fixes for 6.3-rc1 2023-03-03 14:51:15 -08:00
samples LoongArch changes for v6.3 2023-03-01 09:27:00 -08:00
scripts checksyscalls: ignore fstat to silence build warning on LoongArch 2023-03-23 17:18:32 -07:00
security keys: Do not cache key in task struct if key is requested from kernel thread 2023-03-21 16:22:40 +00:00
sound ALSA: hda/ca0132: fixup buffer overrun at tuning_ctl_set() 2023-03-14 17:04:53 +01:00
tools - Add a AMX ptrace self test 2023-03-26 09:01:24 -07:00
usr usr/gen_init_cpio.c: remove unnecessary -1 values from int file 2022-10-03 14:21:44 -07:00
virt KVM/riscv changes for 6.3 2023-02-15 12:33:28 -05:00
.clang-format cpumask: re-introduce constant-sized cpumask optimizations 2023-03-05 14:30:34 -08:00
.cocciconfig
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for *.dtso files 2023-02-26 15:28:23 +09:00
.gitignore kbuild: rpm-pkg: move source components to rpmbuild/SOURCES 2023-03-16 22:45:56 +09:00
.mailmap mailmap: add an entry for Leonard Crestez 2023-03-28 15:24:32 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS There is no particular theme here - mainly quick hits all over the tree. 2023-02-23 17:55:40 -08:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS USB/Thunderbolt driver fixes for 6.3-rc4 2023-03-26 10:22:44 -07:00
Makefile Linux 6.3-rc4 2023-03-26 14:40:20 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.